Today, the microgrid and energy storage industry is increasingly promising. Growing cybersecurity threats and frequent natural disasters pose greater risks to power systems, making microgrid solutions an ideal infrastructure for customers and utilities to improve power supply.
Sometimes, however, the terms "microgrid" and "energy storage" are used interchangeably. This means that energy storage systems can provide backup power, and it is important to recognize that microgrids and energy storage are not the same thing.
Energy storage systems are certainly an essential element for microgrids. Energy storage systems are a flexible and versatile form of distributed energy that can provide significant benefits to microgrids. However, the deployment of energy storage systems alone does not constitute a microgrid, and in many cases, microgrids can be designed and implemented without the deployment of energy storage systems.
Because adding energy storage resources creates additional capital investment for microgrid projects, customers must define their resiliency and energy security goals and work with an energy solutions partner to achieve them. Their partner should be independently owned and operated, be able to offer the best combination of solutions, have extensive experience in energy efficiency and maximizing cost effectiveness, and have the generation, control and storage expertise to effectively develop, design and implement a successful energy security solution.
Microgrid Resilience Scope
It is important to define what kind of microgrid is important to answer the question of how important energy storage systems are to their design.
In the event of a grid outage, a microgrid provides backup power to customers and serves the electrical load on site in a safe and reliable manner, independently. This disconnected state is often referred to as "islanding" because it is effectively a small power system that can meet the needs of customers without the need to transmit power to or from the island. If a site's load cannot be served by on-site generation, then priority needs to be given to providing power to those loads that are considered critical to the site's operations.
Ameresco has introduced an efficient design approach to microgrids that has core features including: energy efficiency, distributed generation, microgrid control, and to some extent, storage of power. This efficiency-centric approach ensures that microgrid systems are optimized for efficiency to minimize energy waste and invest most of the cost effectively in next-generation energy storage and control devices.
Three main questions help shed light on the resilience of microgrids, and these resiliences begin to determine the extent to which all of these core functions are necessary. First, how long does it take for users to complete the transition from grid-connected to islanded status? Second, how much of the total site load is critical? Third, how long does the customer expect to serve these critical loads (e.g., minutes or weeks)?
The microgrid resiliency shown in the figure ranges from inelastic to fully resilient and allows for a physically seamless transition from connected to the grid. Customers without a microgrid deployed (e.g., residential customers) will be completely disconnected during a grid outage. Their power will only be restored once the utility has diagnosed and resolved the grid outage or fault.
Second, customers equipped with on-site emergency backup generation and transfer switches can partially restore power because some or all of their loads can be served depending on the power of the generator. Their power supply will be temporarily interrupted until a backup generator can provide continuous power to the load. In this scenario, some particularly critical loads (such as computers or data servers) will likely use backup power provided by an uninterruptible power supply (UPS) to provide power to the specific critical load for a limited period of time. Many office buildings are examples of such scenarios.
For some customers, interruptions in power supply pose a significant risk to critical business operations, or significant economic loss in the form of reduced productivity, decreased production of products, interruption of research in a university or laboratory environment, or other loss of value. This requires an uninterrupted transition to standalone islanded operation during periods of grid disruption, meaning that power to certain loads within the customer's facility is not interrupted at all.
While a transition with uninterrupted power (shown in the right half of the figure) may be required for many customers, this capability adds complexity and cost to the microgrid. In order to make the transition from uninterrupted to islanded operation and not compromise the continuity of power supply, protective electrical control equipment must be safe and immediately isolate the connection between on-site generation and unstable power facilities, and emergency generation must be immediately available to serve loads and provide on-site generation, such as cogeneration to provide power to loads, in the event of grid disturbances and disruptions. And this can be achieved if microgrid controls are used that can identify and quickly respond to power disturbances and interruptions.
The key decision regarding a successful uninterrupted transition to islanded operations is whether there is enough generation to serve the critical loads at the site, or whether fewer critical loads need to be reduced (shut down) to match the available generation. Microgrid control measures may need to be performed quickly to maintain the stability of the on-site diesel generators and to maintain continuity of power supply.
In order to achieve uninterrupted switching between the grid and on-site generation, a fully resilient microgrid, such as the right-most resilient range, needs to have enough diesel generators to meet all on-site load requirements. This must be implemented in the event of a power outage and for several consecutive hours or days at a time until the grid returns to normal. Even in such cases, microgrid control may be considered in order to respond to unforeseen circumstances, such as diesel generator failure or unexpected changes in load levels.
Microgrids provide power users with the ability to safely disconnect from the grid
An uninterrupted transition without energy storage can be achieved if sufficient generation is available or planned to be implemented, and appropriate microgrid control and the appropriate microgrid control infrastructure are in place. However, adding energy storage systems to a hybrid grid can significantly enhance the microgrid scenarios described above. For customers who do not have a continuous supply of backup power, energy storage systems become a very important aspect of the microgrid architecture.
The role of energy storage systems in microgrid solutions
Modern energy storage systems are unique in that they are very responsive, can both generate and absorb power, and in some cases can regulate the quality of real and reactive power in the distribution system. These capabilities allow energy storage systems to serve a variety of roles within the microgrid, such as customers who need uninterrupted islanded power, do not require on-site generation, or need to supplement the on-site generation present in their distribution system. Learn more below about the role that energy storage systems play in the two distinct operational phases of a microgrid: making the transition from grid-connected to islanded operation simultaneously, or continuing to island.
Effectively making the transition to islanded operation requires a great deal of coordination and very fast control actions, measured in milliseconds. During this time, the high power and fast response capability of commercially proven battery storage systems, such as lithium-ion batteries, can be used to provide effective instantaneous power to the microgrid for a limited period of time, providing power during grid outages until on-site generation equipment can serve the majority of the load for extended periods of time. If not equipped with backup diesel generators, the microgrid can only be maintained until the storage capacity is depleted, which typically ranges from 15 minutes to several hours. Some storage inverters are able to regulate the voltage and frequency within the islanded power system to maintain the power quality of the islanded power system as the grid supply is transformed into an independent islanded power supply. This function is usually fulfilled during normal grid operation or through diesel generators, but if properly designed, it can be accomplished through energy storage systems.
Once the conversion to islanded operation is complete, the role of the energy storage system shifts from short time response to maximizing the operating duration of the microgrid. The charging and discharging capability of the energy storage system creates a dispatchable resource that can follow the instructions of the microgrid control system to provide additional generation or to balance the supply of on-site generation through power demand. Similarly, since the capacity of a current commercial energy storage system typically provides only four hours of backup power, the ability of the storage system to continuously support the microgrid is limited unless it is recharged by on-site generation. Thus the use of energy storage systems without power generation equipment poses a risk to the operational continuity of equipment that requires critical power for extended periods of time.
Perspectives from On-Site Generation
Ameresco's microgrid and energy storage projects are used as examples to illustrate the different roles of energy storage systems in microgrids. Ameresco's approach to energy security is built primarily on developing, designing and implementing solutions that are tailored to the customer's specific site needs and conditions. This is critical to providing cost effective and technically capable solutions to customers, as microgrids and energy storage systems are not a one-size-fits-all concept.
Ameresco showcased its implementation of a microgrid solution without energy storage at the Portsmouth Naval Shipyard in the UK. The project, funded through the UK Ministry of Defense ESTCP program, transitioned the project from grid to silo operation by using new microgrid controls to complement the existing cogeneration and diesel generation (DG) facilities.
Prior to implementation, existing generation assets at the site included two gas turbines and two emergency diesel generating units. However, the site's power demand was often higher than the capacity of these backup sources. During normal operations, the site purchased power from the utility to serve the loads supported by its on-site generation. In contrast, prior to the project, power outages during grid disruptions or load overloads resulted in a complete blackout at the site, and the backup diesel generators were usually not immediately available to restore power when the site's generation equipment could not carry the full load.
The project focused on using an advanced microgrid control system. This system detects when an impending power interruption is coming and very quickly (about tens of milliseconds) shuts down non-critical loads across the site to prevent a more widespread outage. While a large energy storage system would certainly enhance the solution, this project shows that the deployment of energy storage systems is not always a strict requirement for an effective microgrid.
Ameresco, on the other hand, provided a microgrid solution for a customer in the central United States. The erratic power supply of the local grid forced it to use backup diesel generators to serve 100% of its electrical load.
Ameresco provided a solution that used a large solar photovoltaic array and a large battery storage system, along with an advanced microgrid control system, to reduce the backup capacity of this site's conventional diesel generators.
These generation combinations allow this customer to adopt power from the grid, as the solution provided mitigates the original grid reliability issues. When power from the grid is lost, the energy storage system deployed at this site will assume significant responsibility for controlling system voltage and frequency and stabilizing power while the solar power system works until the backup distributed power source is available online. Without a battery storage system as part of this solution, the site is unable to achieve an uninterrupted transition to the microgrid.
Ameresco has implemented and is developing numerous microgrid and energy storage projects where battery storage systems provide customers with significant grid-tie capability. One example is a project at a federal courthouse in California. Ameresco recently provided a 750kW/ 1425Kwh lithium-ion battery storage system, along with a rooftop solar power system and building energy efficiency measures. The battery storage system significantly reduced the cost of electricity demand to the power plant and the utility's peak energy consumption. Electricity cost drivers facilitated the development of this solution rather than energy security.
Conclusion
Deploying an energy storage system does not constitute a microgrid, as generation and control needs to be considered to meet the customer's transition from grid service to islanded operations and to maintain the expected duration of outages on site.
Energy storage systems should be considered as a valuable asset that can bring significant additional benefits to the microgrid. It maintains the power quality of the islanded generation system by providing instantaneous power generation and introducing the ability to absorb and release power. Energy storage systems can act as a glue that binds the microgrid together while generating value during normal grid-connected operation. The speed of power demand response allows the storage system to provide some advantages that the customer's backup generation cannot provide. In short, there is no "one-size-fits-all" solution for customers. Customers considering a power resiliency solution should seek a partner with extensive experience to help evaluate, design, implement and operate a successful solution that integrates their existing assets, risk tolerance, mission, and cost objectives.
